There is a myriad of prior art compressed air supply systems for usage on railway vehicles. For example, Japanese Utility Model Registration No. 57-22979 and Japanese Utility Model Registration No. 54-27603, as well as Japanese Utility Model Registration No. 53-38920, show and disclose a number of different types of compressed air supply systems which were used in the past. Another compressed air supply system is illustrated in FIG. 4 of the present application. As shown, the air supply system 101 includes a compressed air portion 2 and an electrical control portion 103. The compressed air portion 2 is powered by an electric motor 6 which is conected to the main electrical supply circuit 5. That is, the air compressor 7 is driven by the electric motor 6 for charging the air reservoir 10. As shown, the supply path is from the discharge outlet 7a of the air compressor 7 to the input of an intercooler 12 which, in turn, is connected to the discharge pipe 9. The input of a moisture remover is connected to pipe 9 while the output is connected to a check valve 8. Check valve 8 permits air flow from the discharge outlet 7a to the inlet of air reservoir 10. A safety valve 14 is connected between check valve 8 and reservoir 10. A spring-biased, solenoid-operated unloading valve 11 is connected to pipe 9 between the check valve 8 and the air compressor 7. When the solenoid is deenergized, the spring biases the valve 11 to the opened position .tau. so that pipe 9 is connected to the atmosphere, as shown. Now, when the solenoid is energized, it shifts the valve 11 to the closed position .quadrature. which causes the pipe 9 to be blocked off from the atmosphere. At the same time, the air compressor 7 is driven by the energized motor 6 so that compressed air is fed to the air reservoir 10 through the discharge pipe 9 to be used for the operation of the brake system. Preferably, the motor 6 is an induction motor which is simple in construction and is low in price. However, the starting torque of such a motor is small. Therefore, the unloading valve 11 provides that the pressure inside the discharge pipe 9 is low during the start-up operation of the motor 6. The electrical control circuit 103 includes a pressure sensor 15 which has an upper limit pressure portion and a lower limit pressure portion. Thus, the pressure in the air reservoir 10 is sensed so that the main relay 16 opens and closes the main circuit 5. The electric power supply is initially turned ON by the ON-OFF switch 18 and is subsequently opened and closed by the electric contactor 15a of pressure sensor 15. That is, the electric contactor 15a is mechanically linked to the pressure sensor 15, which senses the pressure in air reservoir 10. The electric contactor 15a of the pressure sensor 15 opens the control circuit 103 when the pressure in the air reservoir 10 reaches the upper limit pressure value, and closes the control circuit 103 when the pressure in the air reservoir 10 decreases to the lower limit pressure value. In viewing FIG. 4, and as previously noted, 12 is the cooler, 13 is the moisture-removing device, and 14 is the safety valve. Initially, the air supply system 101 is in a loading mode of operation, since the inside of the air reservoir 10 is at atmosphere. The control circuit 103 is energized by the closure of switch 18 and the closed electric contactor 15a of the pressure sensor 15. When the power supply switch 18 is closed, the main relay 16 and the unloading valve 11 are energized. The energization of solenoid valve 11 shifts to the closed position .quadrature. and the energization of relay 16 closes the contacts of the main circuit 5. This energizes the motor 6 and causes it to drive the air compressor 7. The compressed air from the air compressor 7 passes through the cooler 12, the moisture-removing device 13, and the check valve 8 and is accumulated in the air reservoir 10. As noted above at this time, the unloading valve 11 is in the closed position .quadrature. so that the exhaust pipe 9 is not opened to the atmosphere. Thus, the pressure inside the discharge pipe 9 and the reservoir 10 begins to build up. When the air pressure in the air reservoir 10 reaches the upper limit pressure value, the pressure sensor 15 detects this and the electric contactor 15a is opened. Thus, the control circuit 103 is deenergized. Then, the main relay 16 and the unloading valve 11 are deenergized. Accordingly, the main circuit 5 is opened, and the motor 6 is stopped. At the same time, the air compressor 7 is stopped and valve 11 is spring-biased to the opened position .tau.. At the same time, the compressed air in the discharge pipe 9, which is located between the air compressor 7 and the check valve 8, passes through the unloading valve 11 to the atmosphere.
When the control circuit 103 is opened, the compressed air in the air reservoir 10 is consumed or depleted each time the brake system is operated. This causes the pressure in the air reservoir 10 to be reduced until it reaches the lower limit pressure value. The pressure sensor 15 detects this lower limit pressure value and causes the electric contactor 15a and the control circuit 103 to become closed. Then the main relay 16 and the unloading valve 11 are again energized so that the contacts in the main circuit 5 are closed. The motor 6 starts to run and drives the air compressor 7. The energization of the unloading valve 11 causes it to move to the closed position .quadrature.. However, at this time, the pressure inside the discharge pipe 9 is at atmospheric level so that the initial load on the motor is minimal. However, in such a prior art air supply system 101, the pressure in the discharge pipe 9 will not decrease if the unloading valve 11 is stuck in the closed position .quadrature.. Conversely, the pressure in the discharge pipe 9 will not build up when there is an air leak in the compressed air circuit 2 or when the unloading valve 11 is stuck in the opened position. Now, when the pressure in the air reservoir 10 becomes less than the lower limit pressure value, the motor 6 becomes energized and the compressor 7 attempts to build up the pressure in pipe 9 and reservoir 10. However, with the unloading valve 11 stuck in its opened position .tau., the air is bled off to atmosphere and no buildup occurs. Under this condition, the motor continues to run and is susceptible to burn up. In addition, it is highly uneconomical to keep the motor running continuously.